How to copy file inside jar to outside the jar?

First of all I want to say that some answers posted before are entirely correct, but I want to give mine, since sometimes we can't use open source libraries under the GPL, or because we are too lazy to download the jar XD or what ever your reason is here is a standalone solution.

The function below copy the resource beside the Jar file:

  /**
     * Export a resource embedded into a Jar file to the local file path.
     *
     * @param resourceName ie.: "/SmartLibrary.dll"
     * @return The path to the exported resource
     * @throws Exception
     */
    static public String ExportResource(String resourceName) throws Exception {
        InputStream stream = null;
        OutputStream resStreamOut = null;
        String jarFolder;
        try {
            stream = ExecutingClass.class.getResourceAsStream(resourceName);//note that each / is a directory down in the "jar tree" been the jar the root of the tree
            if(stream == null) {
                throw new Exception("Cannot get resource \"" + resourceName + "\" from Jar file.");
            }

            int readBytes;
            byte[] buffer = new byte[4096];
            jarFolder = new File(ExecutingClass.class.getProtectionDomain().getCodeSource().getLocation().toURI().getPath()).getParentFile().getPath().replace('\\', '/');
            resStreamOut = new FileOutputStream(jarFolder + resourceName);
            while ((readBytes = stream.read(buffer)) > 0) {
                resStreamOut.write(buffer, 0, readBytes);
            }
        } catch (Exception ex) {
            throw ex;
        } finally {
            stream.close();
            resStreamOut.close();
        }

        return jarFolder + resourceName;
    }

Just change ExecutingClass to the name of your class, and call it like this:

String fullPath = ExportResource("/myresource.ext");

Edit for Java 7+ (for your convenience)

As answered by GOXR3PLUS and noted by Andy Thomas you can achieve this with:

Files.copy( InputStream in, Path target, CopyOption... options)

See GOXR3PLUS answer for more details

Given your comment about 0-byte files, I have to assume you're trying to do this programmatically, and, given your tags, that you're doing it in Java. If that's true, then just use Class.getResource() to get a URL pointing to the file in your JAR, then Apache Commons IO FileUtils.copyURLToFile() to copy it out to the file system. E.g.:

URL inputUrl = getClass().getResource("/absolute/path/of/source/in/jar/file");
File dest = new File("/path/to/destination/file");
FileUtils.copyURLToFile(inputUrl, dest);

Most likely, the problem with whatever code you have now is that you're (correctly) using a buffered output stream to write to the file but (incorrectly) failing to close it.

Oh, and you should edit your question to clarify exactly how you want to do this (programmatically, not, language, ...)